The present invention concerns a vehicle tire intended to be fitted with spikes.
Increasing traffic load and spike tyres in combination have proven to be a remarkable road attrition factor. In some countries this has even led to prohibition of spike tyres, or at least to considerable restrictions.
In Nordic conditions, the beneficial effect of anti-slip means on the safety and flexibility of traffic has on the other hand been irrefutably demonstrated, and this effect should not be sacrificed; instead, the associated drawbacks should be eliminated. Good results will be achieved by further developing both the road superstructures and the anti-slip tires.
As a pneumatic automobile tyre rolls on an even surface, it is considerably flattened radially, owing to its flexibility, whereby in the contact region longitudinal as well as transverse forces are generated owing to changes of the rolling radius.
The longitudinal forces acting on the spike when the tire is rolling are due to bending of the body structure, to longitudinal slipping and to the stress wave building up in the rubber.
When a spike approaches the point of contact with the road, the tyre body undergoes bending such that the radius of the bent part is significantly smaller than that of equivalent parts of the load-free tyre. This deflects the spike, which has been mounted at right angles against the surface, to assume a vertical position before contact with the road. Owing to the protrusion of the spike point, however, the spike is not turned sufficiently; it meets the road surface in an oblique position. At this stage, the forces due to slipping tendency also being to exert their influence.
Traditionally, the shape of the spike has been symmetric and it has been mounted in a hole, perpendicular against the wear surface of the tire, whereby it meets the road in a slightly oblique position, as described above. The oblique contact of the spike with the road, as well as its being pushed deeper into the tyre in oblique position during the initial part of road contact, cause damage to the rubber and to the spike which impairs the friction properties of the spike, detracts from the durability of the spike and increases its road attrition properties.